Three figures, three instruments, three questions

A certificate of analysis for a synthetic peptide is not one measurement written down. It is a set of independent determinations, each made on a different instrument, each answering a question the others cannot, and each expressed against a basis that has to be stated before the figure means anything. The line most buyers read first, purity by reversed-phase high-performance liquid chromatography, says least about how much peptide is in the container.

That is not a complaint about the method. Chromatographic purity separates the sample into peaks and reports each peak as a fraction of the total area the detector saw. What it cannot see is anything that fails to absorb at the detection wavelength, or that never comes off the column. Water, counterion and residual inorganic salt each fail one test or both. All of it is in the container, all of it carries weight, and none of it appears in the number.

USP 1503 is the general information chapter on the quality attributes of synthetic peptide drug substances. The European guideline is the document that keeps them apart, and its specification list names them separately: purity, high molecular weight impurities, assay and content, counter-ion identity and content, residual ion content, water content, residual solvents, elemental impurities, bacterial endotoxins and microbiological purity, each with its own method. A certificate that collapses them into a single figure has not made the material easier to judge. It has dropped the other measurements.

Reading the HPLC purity line on a certificate of analysis

Area percent is a ratio of detector response, not a ratio of mass. Response depends on what absorbs. The one chromophore every peptide has in common is the amide bond, and it absorbs in the low ultraviolet, which is why a peptide purity method is normally read near 210 to 220 nanometers rather than at 280, where only tryptophan, tyrosine and phenylalanine answer. A related substance differing from the target by a single residue absorbs almost identically, so area percent tracks mass percent closely for peptide-related impurities and not at all for anything without that chromophore.

What the peaks are matters as much as how many. A synthesis produces peptide-related impurities: deletion sequences missing a residue, truncated and insertion sequences, diastereomers from racemization at a coupling step, and later oxidation and deamidation products. Several differ from the target by so little that they co-elute under an ordinary gradient.

Synthetic peptides sit outside the impurity thresholds most buyers assume apply. The European guideline on the development and manufacture of synthetic peptides is explicit that they are excluded from the scope of ICH Q3A, and points instead at the European Pharmacopoeia general monograph for substances for pharmaceutical use, under which peptide-related impurities are reported above 0.1 percent, identified above 0.5 percent and qualified above 1.0 percent. Where impurities are observed as one co-eluting peak, the same guideline applies the 1.0 percent qualification threshold to the combined peak unless something else is justified.

So a purity figure with no impurity table beneath it is half a measurement. The missing half is the half that says whether the remaining 0.4 percent is one identified deletion sequence or four unresolved things.

A purity figure without its method is a number without a unit

Two laboratories running two gradients on two column chemistries will report two different area percentages for the same lot. The number is a property of the separation as much as of the material, which is why the method belongs with the figure: column chemistry and dimensions; mobile phase and its ion-pairing additive; gradient and flow; column temperature; detection wavelength; injection volume; and the threshold below which peaks were not integrated.

The federal requirement is narrower and points the same way. Under 21 CFR 211.194(a)(2), laboratory records must carry a statement of each method used in the testing of the sample, and the suitability of all testing methods used shall be verified under actual conditions of use. A method transcribed from a supplier's paperwork and never run in the receiving laboratory has not met that sentence. USP 621 is the general chapter chromatographic procedures and their system suitability run under, and a result reported without evidence that system suitability was met has skipped its own precondition.

Purity is also not identity. A wrong sequence assembled cleanly measures high on a purity assay. The European guideline recommends at least two orthogonal methods for identification at release, naming mass, relative retention time, LC-MS, peptide mapping, amino acid analysis and NMR. 21 CFR 211.84(d)(2) sets the terms on which a supplier's report of analysis may be accepted at all: at least one specific identity test conducted by the receiving manufacturer, and validation of the supplier's test results at appropriate intervals.

Net peptide content is a ratio of masses, not of peak areas

This is the difference between net peptide content and HPLC purity, and it is the one that changes what a buyer receives. Purity is a ratio of areas inside a chromatogram. Peptide content is a ratio of masses inside the container. A lot can be 99.6 percent pure and 78 percent peptide at once, and both figures are correct, because they are not describing the same thing.

Content is measured by weight or by counting atoms, never by integrating a chromatogram. The European guideline names, for assay and content, liquid chromatography against a reference standard, elemental analysis, amino acid analysis, nitrogen analysis by Kjeldahl, and quantitative NMR. Amino acid analysis, described in USP general chapter 1052, hydrolyzes the peptide, separates the constituent amino acids and quantifies them, then works back to how much peptide the powder held. It answers a question chromatographic purity was never asked.

The arithmetic is worth doing once.

The counterion is a specification line, not a footnote

Residual trifluoroacetate is not contamination in the accidental sense. It is a consequence of how the material was purified. Preparative reversed-phase chromatography of peptides is usually run with an acidic ion-pairing additive, most often trifluoroacetic acid, and the peptide leaves the lyophilizer as the salt of that acid. Reaching an acetate or chloride salt takes a deliberate exchange step, and a certificate silent on the counterion is not evidence that the step happened.

The European guideline makes it a control point rather than a curiosity. Its specification list carries counter-ion identity and content as one line and residual ion content, naming TFA, as a separate one, and its footnote is unambiguous: the type of counter ion should be defined, and the amount of counter ions should be controlled in the active substance specification with a justified upper limit.

Measuring it is routine. The guideline's characterization table gives liquid chromatography and ion chromatography for the identity of counter ions, the European Pharmacopoeia carries a dedicated general method for the acetate case, 2.5.34, acetic acid in synthetic peptides, run on an octadecylsilyl column at 210 nanometers, and fluorine NMR quantifies trifluoroacetate directly.

The consequence reaches the label. The same guideline says the strength of a finished product should be defined with respect to the mass of peptide base, not including salt or counter-ion, and that limits of assays determined by liquid chromatography are expressed in terms of the counter-ion free, anhydrous substance, unless otherwise justified. An as-is assay and a counterion-free, anhydrous assay are different numbers about the same lot, so two certificates that disagree about the basis cannot be compared. On the arithmetic above, the gap between gross powder and peptide base is roughly a fifth.

Endotoxin, and the figures whose basis the supplier does not hold

An endotoxin figure is a measurement, not a verdict. USP 85 is the compendial test, and the limit a parenteral material is judged against is not a fixed number: it is K divided by M. FDA sets K at 5.0 endotoxin units per kilogram of body weight, and at 0.2 where a product is labeled for the intrathecal route. M is the largest quantity, per kilogram, that would be given in a single hour.

M is not the supplier's to hold. It belongs to the prescriber and to the pharmacy preparing the finished article, so a supplier can report the figure, its unit and its method, and cannot report a pass. A certificate reporting endotoxin in units per milligram has also handed the reader an arithmetic problem, because converting to a per-container figure uses the peptide content, not the gross weight of the powder.

Water is the quieter version of the same point. Karl Fischer determination under USP 921 measures what the lot held at release, and peptides are hygroscopic powders, which the European guideline says plainly when it asks that reference standards be protected against moisture uptake. The water figure is a release measurement with a storage condition attached, and that condition is part of the specification rather than advice.

What this means for a buyer

Four figures and their stated bases settle what a container holds, and none of them settles it alone. The purity line is the cleanest number on the page because it is a ratio of areas inside a well-behaved separation, and that cleanliness is precisely why it cannot be read as mass.

So the request to make of a supplier is specific rather than general.

Statute already assumes most of this. Under 21 U.S.C. 353a a bulk drug substance used in compounding has to clear three separate conditions: it complies with an applicable United States Pharmacopeia or National Formulary monograph where one exists, and where none exists it is a component of an approved drug or appears on the list FDA develops by regulation; it is manufactured by an establishment registered under section 360; and it is accompanied by a valid certificate of analysis. The certificate is the instrument that releases the material, not a courtesy document. Every entry on the GradeBio register prints the floor its material is released to, by HPLC area percent, because a floor is a commitment and a typical figure is not. The floor is still one number. Reading the other three is what tells a buyer what is in the vial.

• The method behind the purity figure, in enough detail to repeat it: column, mobile phase and additive, gradient, flow, temperature, detection wavelength, and the reporting threshold.
• The impurity table under the purity figure, with identified impurities named and unresolved peaks declared as unresolved.
• The assay with its basis printed beside it, as is or counterion free and anhydrous, so two certificates can be compared at all.
• The counterion by name, its measured content, and the upper limit the lot was released against.
• Water content with its storage condition, and endotoxin with its unit and its method.
• The identity work and not only the purity work: at least two orthogonal methods, and an identity test run by the receiving laboratory.

Sources

1. European Medicines Agency, Committee for Medicinal Products for Human Use and Committee for Veterinary Medicinal Products Guideline on the Development and Manufacture of Synthetic Peptides EMA/CHMP/CVMP/QWP/367182/2025
2. United States Pharmacopeia General Chapter 1503, Quality Attributes of Synthetic Peptide Drug Substances
3. United States Pharmacopeia General Chapter 1052, Biotechnology-Derived Articles, Amino Acid Analysis
4. British Pharmacopoeia, Appendix VIII Q, stated on its face to correspond to European Pharmacopoeia general method 2.5.34 Acetic Acid in Synthetic Peptides
5. United States Pharmacopeia General Chapter 85, Bacterial Endotoxins Test
6. United States Food and Drug Administration, Inspection Technical Guide Bacterial Endotoxins/Pyrogens
7. Code of Federal Regulations, Title 21 21 CFR 211.194(a) and (a)(2)
8. Code of Federal Regulations, Title 21 21 CFR 211.84(d)(1) and (d)(2)
9. United States Code, Title 21 21 U.S.C. 353a(b)(1)(A)
10. United States Pharmacopeia General Chapter 621, Chromatography